Conventional electrical power systems utilize a synchronous electrical generator for generating AC power. Particularly, such a generator may include a rotor and a stator having a stator coil. In applications such as to an aircraft, the rotor is driven by an engine so that electrical power is developed in the stator coil. Owing to the variation in engine speed, the frequency of the power developed in the generator windings is similarly varying. This varying frequency power is converted to constant frequency power in a variable speed constant frequency (VSCF) system including a power converter which may develop, for example, 115/200V.sub.ac power at 400 Hz. Such known converters are controlled by a generator/converter control unit (GCCU).
In order to provide aircraft engine starting, such known power systems have operated the generator as a motor. Specifically, an external power source is coupled through a start control to the generator to energize the stator coil and thus develop motive power to rotate the engine and thus start it. The components required in such a start control increase the weight of the aircraft and take up valuable space. To minimize the size and weight of such added start controls, certain known aircraft VSCF power systems have utilized the existing converter and GCCU both for the start and the generate functions.
VSCF generators typically include a permanent magnet generator which is coupled through a regulator to a DC winding of an exciter. The exciter includes an armature winding which typically develops polyphase AC power which is rectified and supplied to the DC field of the main generator. However, because the exciter uses rotation to transform mechanical power into electrical power, the excitation for the wound field main generator cannot be supplied at zero speed. A rotary transformer, having a secondary winding rotating with the common shaft, can be used to provide excitation for the wound main generator field even at standstill. A rotating transformer would utilize high frequency AC power, supplied by an external source, to energize its stationary winding, which would induce an AC current in its secondary winding. This AC coupled power could then be rectified, as above, to provide the DC field power to the immobilized main generator field winding. Such a rotating transformer configuration would require the use of an additional device in the power system attached to the common shaft. The use of an additional rotating transformer for starting the engines would not only increase the weight of the aircraft, but also unduly occupy valuable space, and require additional cost.
One approach for overcoming these problems is disclosed in Shilling et al. U.S. Pat. No. 4,743,777 which discloses a starter/generator system including an exciter having distributed AC and DC windings both carried in exciter stator slots. Such a construction, however, exhibits substantial pole-to-pole flux leakage between adjacent slots which create the magnetic poles. When using the DC field winding in the generate mode, this flux loss can significantly degrade the efficiency of the exciter.
The present invention is intended to overcome one or more of the problems as set forth above.